It is often desirable to minimize the intrusion of water through a louver. In such cases, the use of a drainable-blade louver, many forms of which are available commercially, is called for. Common to almost all drainable blade louvers is the provision of troughs on the front edges of the upper surfaces of the blades for capturing water that would otherwise run down the blades and fall from the front edges onto the blades below. In so doing, some of the water dripping from the front edges of the blades may become entrained in the air flow and carry through the louver. The troughs capture the water and drain it to vertical drainage channels in the jambs of the blade frames. Some drainable louvers have offsets in the blades to stop water from being blown up the blade surfaces and off the upper, rear edges. Other drainable blade louvers, such as the one described and shown in U. S. Pat. No. 4,103,468 (Olsen, Aug. 1, 1978), have additional troughs intermediate the front and rear edges of the blades and an upper rear flange for capturing water from splash and water blown up the blade surfaces. At least one commercially available drainable blade louver, which is of the sightproof type with inverted V-shaped blades, has troughs on the bottom sides of the rear edges of the blades to catch water carried by centrifugal force of the air flow onto the undersides of the back portions of the blades.
Windblown rain enters the front of a louver and impinges on the blades with considerable force. Water drops that strike the blades splash into small droplets, some of which become entrained in the airflow and some of which are carried up, either by their momentum or by rising turbulent air currents, and wet the underside of the blade above. Some of the droplets fall back onto the blade surface. Droplets that are captured by the blade surfaces and collect on the blade surfaces tend to coalesce into a layer of water. The air flow tends to push the water layer formed from the droplets up the blade surfaces.
Previously known drainable blade louvers vary in their effectiveness in preventing water intrusion (carry-through). Most drainable blade louvers, for example, make no provision for capturing water caught on the undersurfaces of the blades. Many lack sufficient drainage trough capacity to drain off all water captured by the troughs in heavy storm conditions, and the troughs overflow. Previously known drainable blade louvers also make no provision for promoting the formation of large, heavy drops from droplets that collect of some surfaces, and the droplets on such surfaces are prone to being picked up and entrained in the air flow.